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The Kepler connection
03.31.09

NASA's plans to detect Earth-like planets around stars with Kepler, SIM Lite, and possibly coronagraphic imaging and formation flying interferometer spacecraft (artists' concepts of missions shown above).

The Kepler mission, launched on March 6, 2009, is the first step in NASA's quest for finding Earth-like planets with space-based telescopes. Kepler will monitor 100,000 stars for the telltale dimming that will occur when an Earth-like planet passes in front of its parent star and blocks out a small portion of its star's light. The depth and duration of that eclipse will give information about the orbital period of the planet and its size. The larger the planet, the more light it will block from its star, also the closer the planet orbits its star, the faster it will complete its orbit and so the more eclipses Kepler will be able to observe. There are a minimum of 3 eclipses that Kepler must see to make sure that the planet is real. Kepler's limitations are that it will not be able to see eclipses from planets that are orbiting further out from their stars, and hence orbit slower, since they will not make enough eclipses in the 3.5 year lifetime of the mission. Still, by studying such a large number of stars, Kepler will provide the statistics of how common Earth-like planets are around stars similar to our sun.

The second step in finding Earth-like planets after Kepler is the SIM Lite Astrometric Observatory, currently in development Phase B. Unlike Kepler, SIM Lite will be monitoring stars for the telltale wobble induced by the orbit of a planet on its parent star. The advantages of this method over the eclipsing method is that it doesn't rely on the chance alignment of spacecraft, planet, and star and the shift in the parent star's position in the sky is the cumulative effect of all the planets that orbit it. Therefore, SIM Lite will be able to detect and characterize multiple planets, both Earth-like and Jupiter-like, around a single star and hence give us a full picture of what these other stellar systems looks like. By accumulating statistics on these multiple planet systems we can get a sense of the stability of such systems. If there are a lot of multi-planet systems, then those systems must be stable; if not, then multi-planet systems must not be very stable, implying that our own solar system is a rarity.

Unfortunately, the stars around which Kepler will find planets will be too far for SIM Lite to follow-up, but it will be the statistics established by Kepler that will determine the strategy and number of nearer stars that the SIM Lite mission will monitor for wobbling. The closer a star is, the easier it is for SIM Lite to detect its wobble, especially if the planet's mass is small like the Earth's. If Kepler determines that Earth-like planets are common, then SIM Lite will concentrate its efforts on looking for Earths around every suitable nearby star, confident that there is a high probability that such planets will be found. If Earth-like planets are rare, then to expand the sample and the probability of discovery, SIM Lite will have to monitor a much larger number of stars which will include many that are further away. That will make finding Earth-like planets more difficult and will most likely reveal planets that are a bit more massive than the Earth.

The wobble technique has another advantage. Once SIM Lite identifies a planet around a star, in addition to its orbit, it determines the planet's mass. Then independent imaging techniques can be used to ascertain the sizes of those planets. Combining the size and mass of a planet yield the planet's density and surface gravity.

Knowing these two facts is very important for the next phase of studying Earth-like planets around stars: spectroscopy. If SIM Lite shows that a planet is (1) rocky and (2) has enough mass to hold onto an atmosphere and that (3) it orbits in the habitable zone of its parent star, a future space-based mission can take a spectrum (spread the light into its component colors) of that planet and its atmosphere to see if there might be signs of life!

The job of later missions will not be to search for Earth-like planets but to follow-up on those that have already been found by SIM Lite and tell us the properties of those planets by taking images and spectra. At the moment several follow-up missions are in the study phase, but the capabilities that will be necessary to characterize planets found by missions like SIM Lite are already well known. It is that final goal of getting the chemical signatures of life on an alien world that is the driving force behind the planet searches that NASA has undertaken.

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